This invention relates to a process for manufacturing an Fe-based sintered alloy which is superb in machinability, and more particularly to a process for manufacturing an Fe-based sintered alloy which can be smoothly integrally machined together with such soft material as aluminum. Also, the present invention relates to an Fe-based sintered alloy manufactured through such a manufacturing process, and a bearing cap to be made using such an Fe-based sintered alloy.
An Fe-based sintered alloy is low in manufacturing cost and superb in properties such as strength, wear resistance, etc. For this reason, the Fe-based sintered alloy is used in a wide variety of technical fields. For example, mechanical parts made of an Fe-based sintered alloy, such as a valve driving system, a bearing, and the like, of automobiles, motorcycles, and the like are widely employed because a machining operation can be omitted considerably even for the parts having complicated configurations. However, since many of the mechanical parts made of an Fe-based sintered alloy are nevertheless required to be machined, poor machinability can still become a shortcoming involved in the conventional Fe-based sintered alloy.
In order to improve machinability of the Fe-based sintered alloy, many attempts have heretofore been made. In one attempt, iron powder containing sulfur is used as starting material powder. In another attempt, a sulfide is added to and mixed with starting material powder. In still another attempt, a compact is subjected to sulfidizing treatment in the atmosphere of hydrogen sulfide gas. In an attempt where sulfur as a free-cutting component is dispersed in a matrix of a sintered alloy, improvement of machinability is limited. Further, since sulfur is an element for decreasing strength, particularly toughness, of a sintered alloy and also for promoting corrosion of a sintered alloy, use of the sintered alloy is limited.
Another technique is also provided in which pores in a sintered alloy are filled with resin, or the like. In such a sintered alloy, since resin in the pores serves as an initiating point for chip breaking, chip breaking property is good. However, in such a technique, the kind of resin to be used can shorten the service life of a cutting tool such as a cutter. Moreover, the use of a sintered alloy sometimes requires a process for removing resin from the pores after the cutting operation.
In view of the above, in Japanese Patent Unexamined Publication (KOKAI) No. 79701/95, there is disclosed a technique in which boron nitride is used as a free-cutting component, instead of sulfur. According to this Publication, by adding boron nitride to a starting material power and mixing them together, frictional resistance between a sintered alloy and a cutting tool can be reduced and machinability can be enhanced. In Japanese Patent Unexamined Publication (KOKAI) No. 305147/95, there is disclosed a technique in which cubic boron nitride is added to and mixed with a starting material powder of an Fe-based sintered alloy. According to this Publication, boron nitride prevents diffusion of graphite, which is added as a starting material in a sintered alloy, and forming of pearlite, and promotes graphite to remain as free graphite, which is a cutting-free component.
Recently, aluminum alloy was widely used in parts of automobiles. For this reason, there are many occasions where Fe-based sintered alloy and aluminum alloy parts are integrally machined. Therefore, Fe-based sintered alloy is demanded to have the same degree of machinability as aluminum alloy. However, it is impossible to manufacture an Fe-based sintered alloy having such machinability even in accordance with the above proposed techniques. According to the teaching of the above proposed techniques, since powder for enhancing machinability is added to a starting material powder, the demand for enhancing the machinability of only the part of the sintered alloy which part is to be machined, cannot be satisfied.
With respect to this point, International Publication WO90/12124 and Japanese Patent Unexamined Publication No. 342783/92 disclose a technique in which the alloy surface is subjected to masking treatment when the alloy is subjected to carburizing treatment, so that carburizing is partly prohibited. However, the technique proposed in those Publications is directed to prevention of deterioration of toughness due to carburizing and therefore, such machinability as mentioned above is unobtainable.